Magnetic annealing of ferromagnetic thin films using induction heating

ABSTRACT

A method of performing magnetic annealing of a ferromagnetic thin film applied to a substrate includes applying an oscillating magnetic field to the ferromagnetic thin film to induce a current therein and heat the ferromagnetic thin film. A directional magnetic field is applied to the ferromagnetic thin film at the same time as the ferromagnetic thin film is heated, and the ferromagnetic thin film is allowed to acquire a desired magnetic characteristic, and then the oscillating magnetic field is removed and the ferromagnetic thin film is allowed to cool.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 120 to patentapplication Ser. No. 10/376,497, filed on Feb. 28, 2003, whichapplication claims priority under 35 U.S.C. § 119 to provisional patentapplication No. 60/360,667, filed on Mar. 1, 2002. The entire contentsof both applications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to induction heating, and magneticannealing using induction heating.

Ferromagnetic thin films are important components of manymicroelectronic devices and structures. For example, magnetic randomaccess memories (MRAMs) are magnetic storage devices that storeinformation in the form of magnetization of a ferromagnetic thin filmthat comprises part of a magneto-resistive sensor. Information can beenwritten into the ferromagnetic thin film and read out by detecting amagneto-resistance of the sensor. MRAMs have been refined to allowincreasingly large amounts of data to be stored in a non-volatile mannerand are typically fabricated on wafers using conventional semiconductorfabrication techniques.

The deposited ferromagnetic thin films in MRAMs and other devices andstructures often require magnetic annealing in order to impart certaindesired magnetic characteristics to such a layer. Specifically, magneticannealing refers to a process by which a ferromagnetic materialundergoes exposure to an external magnetic field at elevatedtemperatures in order to increase the size of magnetic domains toincrease permeability and to impart a particular magnetic orientation tothe magnetic dipoles in the ferromagnetic material. The crystallinestructure of a deposited ferromagnetic thin film is responsive toincreased temperature. In particular, raising the temperature increasesthe vibrational moments of atoms forming the crystalline structure,imparts a randomness to the motion of these atoms, and places theseatoms in a state that provides minimal resistance to the influence of anexternal magnetic field. An external magnetic field causes the magneticdipoles of these atoms to be oriented along the axis of the appliedmagnetic field. Prior art systems for performing magnetic annealinginclude a vacuum oven or furnace or the like for providing the elevatedtemperatures required.

Other modern microelectronic circuit process technologies incorporatedevice structures having high sensitivities to thermal treatment. Thehigh sensitivities are due to the precise definition of device regions.These regions may include ultra-thin ion implanted source and drainregions in a submicron complementary metal-oxide-semiconductorfield-effect transistor circuit (CMOS FET), among others, and exposureof such regions to high temperature result in the degradation of deviceperformance. Thus, material and process related temperature limitationsprohibit the integration or incorporation of a wide range of possibledevice structures in the fabrication process. Applications such as hightemperature treatment of embedded high voltage, high current, or highpower microelectronic devices often require complete thermal isolationof process modules. An example of such a system is an embedded processorthat is responsible for driving motors. The system might include driversfor driving the motors, and a plurality of high density logic circuitsfor controlling the operation of the motor. Both the drivers and thelogic circuits typically have different thermal capacities and,therefore, complicated processes are required to embed the drivers andthe logic circuits in a single system.

The incorporation of devices into mainstream processing also presentsthermal challenges for integration of the technologies. Device examplesinclude micro-electromechanical systems (MEMS), micromachines andmicrosystems. Various thin film structures for the devices may alsorequire thermal treatment to stabilize the mechanical properties foruse. However, a microelectronic process typically requires lowertemperatures throughout its processing. The relatively high thermalenergy generated in the thermal treatment of the MEMS tends to impactthe CMOS processing of the system. Similarly, embedding elements, likeradio-frequency components into high density CMOS, require differentthermal treatments and, thus, require complicated processes to embedthem together.

Furthermore, many device packaging applications require protection orpackaging, often prior to final encapsulation or before beingsingulated, in order for the devices to function. Pressure sensors,accelerometers, optoelectronic device assemblies, and somemicroelectronics technologies require packaging or assemblies thatutilize temperatures above 300-400° C., which temperatures could damagemicroelectronic circuitry.

SUMMARY OF THE INVENTION

In one embodiment, the invention provides a method of performingmagnetic annealing of a ferromagnetic thin film applied to a substrate.The method comprises applying an oscillating magnetic field to theferromagnetic thin film to induce a current in the ferromagnetic thinfilm thereby heating the ferromagnetic thin film. A directional magneticfield is applied to the ferromagnetic thin film at the same time as theferromagnetic thin film is heated. The ferromagnetic thin film isallowed to acquire a desired magnetic characteristic and then theenergization of the coil is ceased and the ferromagnetic thin film isallowed to cool.

In another embodiment, the invention provides a method of performingmagnetic annealing of a ferromagnetic thin film. The method includesapplying a ferromagnetic thin film to a substrate and energizing a coilwith an AC source at a predetermined frequency, power level and durationto generate a magnetic flux that induces a current in the ferromagneticthin film thereby heating the ferromagnetic thin film. A magnetic fieldis applied to the ferromagnetic thin film at the same time as theferromagnetic thin film is heated, thereby allowing the ferromagneticthin film to acquire a desired magnetic characteristic.

In another embodiment, the invention provides a method of performinglocalized heating of a first thin film on a substrate. A ferromagneticthin film is applied to the substrate. A first act of applying anoscillating magnetic field to the ferromagnetic thin film is performedto induce current in the ferromagnetic thin film thereby heating theferromagnetic thin film. A magnetic field is applied to theferromagnetic thin film at the same time that the ferromagnetic thinfilm is heated by the first act of applying an oscillating magneticfield, thereby magnetically annealing the ferromagnetic thin film. Asecond act of applying an oscillating magnetic field to theferromagnetic thin film is performed to induce current in theferromagnetic thin film thereby heating the ferromagnetic thin film andthe first thin film to change a property of the first thin film.

Other features and advantages of the invention will become apparent byconsideration of the detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a system.

FIG. 2 is a sectional view of the system shown in FIG. 1 with anelectrically conductive film.

FIG. 3 is a sectional view of the system shown in FIG. 2 placed near aninductive coil.

FIG. 4 is a table listing the inductive heating properties of selectedmicrosystem materials and some common ferromagnetics.

FIG. 5 is an isometric view of a second system containing a heatingelement.

FIG. 6 is a sectional view of a system for performing magneticannealing.

DETAILED DESCRIPTION

Before any embodiments of the invention are explained in detail, it isto be understood that the invention is not limited in its application tothe details of construction and the arrangement of components set forthin the following description or illustrated in the following drawings.The invention is capable of other embodiments and of being practiced orof being carried out in various ways. Also, it is to be understood thatthe phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including”, “comprising”, or “having” and variations thereof herein ismeant to encompass the items listed thereafter and equivalents thereofas well as additional items. Unless limited otherwise, the terms“connected”, “coupled”, and “mounted” and variations thereof herein areused broadly and encompass direct and indirect connections, couplings,and mountings. In addition, the terms “connected” and “coupled” andvariations thereof are not restricted to physical or mechanicalconnections or couplings.

The following description relates to methods of and apparatus forperforming regional heating of a substrate by induction heating.Induction heating can be utilized in industrial processes to modify themechanical properties of materials. As an alternative to ambientheating, induction heating allows for localized temperature control of aspecific region of a material, device, or substrate. Thus, not only doesinduction heating potentially conserve energy but it also allowsprotection of temperature-sensitive components. For these and otherreasons, induction heating has several potential applications inmicroelectronic and microsystem fabrication. In one such application,induction heating is used in a magnetic annealing process, as furtherdescribed below.

Inductive coupling to ferromagnetic films allows for the localizedheating of selected areas. This potentially prevents significant heatenergy from reaching the substrate and, thus, allows for substantiallyincreased flexibility in microelectronic and microsystem design.

This description pertains to fabricating thin film structures anddevices on a substrate, and to enclosing a product, such as anintegrated microsystem, in a package. Package examples includemonolithic packaging wafer scale packaging. In one embodiment, theproduct is fabricated on a flat silicon substrate. However, othersubstrate materials can also be used. For example, a substrate can be asemiconductor such as germanium or gallium arsenide, semimetals such asbismuth and molybdenum, metals and metal alloys such as copper andstainless steel, insulators such as silicon dioxide and aluminum oxide,organic polymers such as Teflon, and inorganic polymers such assilioxane. Various different substrate profiles can be used including,but not limited to, flat, curved, cylindrical, and spherical.

Induction heating is applicable to a variety of device classificationsincluding, but not limited to, microelectronic devices,micro-electromechanical devices, and packaging and wafer bondingstructures. FIG. 1 shows a sectional view of a system 100, such as awafer, including a substrate 102, a plurality of devices 105 and 110,and a plurality of structures 115 and 120. A diffusion barrier 122 isdeposited onto devices 105 and 110 as a passivation layer for thesubstrate 102. The first device 105 depicts an electronic device thatdoes not require induction heating, the second device 110 depicts anelectronic device that requires induction heating, the first mechanicalstructure 115 depicts a mechanical structure that requires heattreatment, and the second structure 120 depicts a mechanical structureintended for packaging or wafer bonding.

FIG. 2 shows a sectional view of the system 100 shown in FIG. 1 with anelectrically conductive film 150. The film 150 covers only the areas ofthe system 100 to be heated. In one embodiment, the film 150 ispatterned through photolithography. However, patterning may also beachieved by other methods depending on the type of film applied and thedeposition technique chosen. In some embodiments, a suitable barrierfilm material 124 including, but not limited to, silicon dioxide orsilicon nitride isolates the film 150 from the structural and devicelayers 115, and 120.

In some applications, the system 100 can have more than one layer ofconductive film. That is, the thickness of the film 150 can vary.Additionally, multiple, different film materials can be deposited on orapplied to the system 100. Varying the thickness or varying the type ofthe film 150 allows for different temperature increments when energy isinduced in the film 150.

In one embodiment, the deposition technique is sputtering. However,other techniques suitable for depositing the film 150 on or applying thefilm 150 to the system 100 are available. Other film application ordepositing techniques include, but not limited to, thermal evaporation,liquid phase chemical technique, gas phase chemical process, glowdischarge process, electronic beam (“EB”) evaporation method, ion beamassisted deposition, chemical vapor deposition (“CVD”), plasma enhancedchemical vapor deposition (“PECVD”), pulsed laser ablation (“PLD”),chemical solution deposition, cathodic arc deposition, and a combinationof different techniques or processes. In some embodiments,electroplating is used even though it may result in relatively thickfilm application or deposition. In one embodiment, the thickness rangesfrom a half micron up to ten microns thick. However, depending on theapplication, localized inductive heating still works with otherthicknesses such as 50, 100, 200, 500, or higher number of microns.

Referring now to FIG. 3, the system 100 is placed near an inductive coil170, whose structure, orientation, and distance from the wafer 100 arearranged so as to achieve a desired magnetic coupling effect(represented by 160). The system 100 and the coil 170 are thenpositioned in a substrate chamber 174 to control the inductionenvironment. The magnitude, polarity, and overall behavior of theelectric field induced in the material or materials through which themagnetic field passes define the magnetic coupling effect. In oneembodiment, the system 100 is positioned such that the coil 170 extendsabove and below the substrate by approximately equal amounts so as to beexposed to a magnetic field which is fairly uniform and unidirectional.Specifically, the substrate 100 is positioned adjacent the coil 170 suchthat the nearest loops of the coil 170 are located at a distance of 0.25cm from the top and bottom surfaces of the substrate 100. However, otherconfigurations and distances are possible depending on the coil 170 andthe substrate. A power source applies a time-varying electric field at afrequency and power setting to the inductive coil 170. This results in acurrent being generated within the film 150. The magnitude of thecurrent is a product of the electric field strength and the materialconductivity, σ. A magnetic field of the same frequency is obtainedaccording to Ampere's law, which is given by $\begin{matrix}{{\nabla\quad{\times H}} = {J + \frac{\partial D}{\partial t}}} & (1)\end{matrix}$where H denotes the magnetic field vector, J is the current passingthrough the coil 170, and D is the electric displacement current thatarises due to the stretching of electron clouds in the external mediumand is proportional to the electric field strength by the electricpermittivity of the medium, ε. Placing the charged inductive coil 170near the wafer 100 induces an electric field of the same initialfrequency in the conductive film 150 according to Faraday's law, whichis given by $\begin{matrix}{{\nabla{\times E}} = {- \frac{\partial B}{\partial t}}} & (2)\end{matrix}$where E denotes the secondary electric field vector and B is themagnetic flux. The magnetic flux, B, is proportional to the magneticfield strength by the magnetic permeability, μ, of the film 150. If thesecondary material, such as the film 150, is an electric conductor, thenthe induced electric field gives rise to eddy currents throughout theregion of magnetic field overlap. The result is the heating of thedevice 110, and the structures 150 and 120 through their I²R power loss.Equation (2) indicates that the direction of the alternating currentsoppose that of the conduction current in the coil 170, J. Heating isalso obtained as a result of hysteresis, which occurs as a result of anonzero time required for the material to magnetize along a givendirection. Hysteresis losses are typically less prevalent than eddycurrent losses and increase with frequency.

As for the conductive material used for the film 150, most substancesthat conduct an electric current are capable of being inductivelyheated. As indicated by Faraday's law, however, certain materialsinteract with magnetic fields more effectively than others. The magneticpermeability of a material is largely a function of its unpairedelectrons. When electrons accumulate in the valence band of an atom ormolecule, it is energetically favorable for them to assume states withthe same spin direction until all such states are filled, at which pointthey begin to fill states of the opposite spin direction, formingspin-up and spin-down pairs. If one or more unpaired electrons remain,the material has an overall spin magnetic moment that is polarizablealong any direction by applying a magnetic field. The magnitude of thismoment determines the permeability of a material and its relativeusefulness in inductive heating applications. Substances classified asdiamagnetic have no unpaired electrons and thus are only weaklypolarizable, less so than free space. Paramagnetic materials have atleast one unpaired electron per atom or molecule and thus a magneticfield results in a moderate degree of alignment. Perhaps the mostversatile materials are the ferromagnetics, which can be polarized tovarying degrees by adjusting the strength of the applied field and canoften be made to polarize very strongly. Ferromagnetic materialstypically hold their alignment for a long but finite amount of timeafter the field has been removed, and typically consist of the elementsiron, nickel, and cobalt. Substances from any of these threeclassifications can be inductively heated, and combinations of materialsfrom one or more groups can be used to achieve different degrees of heatgeneration. In one embodiment, a single film of ferromagnetic materialincluding, but not limited to, a permalloy or other iron-containingferromagnetic alloy is heated.

The electrical conductivity and magnetic permeability of a secondarymaterial are also important in that they determine the frequency atwhich it can best be inductively heated as dictated by theelectromagnetic skin effect. The eddy current density decreases fromthat induced at the outer edge of the material according toJ _(r) =J ₀ e ^(−r/δ)  (3)where J_(r) is the current density at a distance r from the surface, J₀is the current density at the surface, and δ is the skin depth. The skindepth, δ, for a good electrical conductor is given approximately by$\begin{matrix}{\delta = \frac{1}{\sqrt{\pi\quad\sigma\quad\mu\quad f}}} & (4)\end{matrix}$where σ is the electrical conductivity, μ is the magnetic permeability,and f is the chosen frequency for modulating the skin depth with respectto the dimensions along which the eddy currents propagate in order toachieve efficient inductive heating. Furthermore, assuming that thedevices 115, 120 on the substrate 102 face out of the coil 170 such thatthe eddy currents propagates tangentially along a coil plane, the powerdissipated, P, in a cylindrical-shaped object with radius a andthickness t is determined by the following piecewise expression:$\begin{matrix}{P = \left\{ \begin{matrix}{{\frac{2\quad\pi\quad H_{0}^{2}t}{\sigma}\left( \frac{a}{\delta} \right)^{4}},} & {a < {1.5\quad\delta}} \\{{{\frac{8\quad\pi\quad H_{0}^{2}t}{\sigma}\left( \frac{a}{\delta} \right)},}\quad} & {a > {1.5\quad\delta}}\end{matrix} \right.} & (5)\end{matrix}$where H₀ is assumed to be the uniform magnetic field measured in thefree interior of the coil 170. Equations (4) and (5) show that inductiveheating increases with frequency according to one of two schemes. Thefirst scheme occurs at low frequencies where the power varies with f².The second scheme occurs at higher frequencies where it levels off tofollow f^(1/2). The process is optimized when the frequency is chosensuch that the a/δ ratio is approximately four, with further increases infrequency producing relatively little benefit. The process can thereforebe configured such that the targeted material or materials are in anoptimal state while the non-targeted material or materials are not.

FIG. 4 gives a comparison of the inductive heating properties of somecommon ferromagnetics as well as several important materials in thefields of microelectronics and Microsystems in a table 400 usingEquations (4) and (5). The spatial values chosen to represent thesemiconducting materials were those of a standard four-inch substrate,where a=50 mm and t=0.5 mm. The values used for the metals correspond tothe size of an inductively heated thin film patterned to cover a typicalinertial sensing structure, a=0.5 mm and t=1 μm. The normalized powerdissipation values show that the ferromagnetics are capable ofconverting magnetic energy to thermal energy far more efficiently thanany of the microsystem materials. In general, the dimensions of themicrosystem metals will be considerably smaller than the values used inthis comparison, further decreasing their ability to absorb magneticenergy, and the ferromagnetics will exhibit additional heating due tohysteresis.

Depending on the nature of the heated object and the conditions underwhich it is heated, induction heating involves the heat transfer modesof conduction, convection, and radiation to varying degrees. Inconduction, heat energy moves within the device 110 and the structures115 and 120 from a region of high temperature to a region of lowtemperature. The rate at which the energy moves depends on a temperaturegradient and is given by Fourier's law, written asq _(cond) =−λ*∇T  (6)where q_(cond) is the conductive heat flux, λ is the spacing thermalconductivity, and T is the temperature. A large temperature gradient canarise through the material cross section due to the skin effect asdescribed, and thus a large thermal conductivity is desirable if theentire material is to be heated with a high degree of uniformity. Theconvective and radiative heat transfer modes both refer to energydissipated at the surface of the device, and are a function of thedifference between the surface temperature and the ambient temperature.Convection is the process by which heat energy escapes from an object toits surroundings and is described by Newton's law asq _(conv)=α(T _(S) −T _(A))  (7)where q_(conv) is the convective heat flux, α is the surface heattransfer coefficient, T_(S) is the surface temperature, and T_(A) is theambient temperature. This is typically an undesirable effect as itcauses energy to be lost to the surroundings rather than applied toheating the material. As suggested by Newton's law, the rate ofconvective heat loss can be reduced by increasing the ambienttemperature or by using an ambient gas that reduces the surface heattransfer coefficient. In one embodiment, the heating of the film occursunder vacuum so as to reduce the number of ambient molecules present toabsorb heat energy.

In another potentially undesirable effect, radiative energy loss occurswhen the temperature difference at the surface causes energy to escapein the form of electromagnetic waves. This is expressed by theStefan-Boltzmann law asq _(rad) =k _(B) e _(s)(T _(S) ⁴ −T _(A) ⁴)  (8)where q_(rad) is the radiative heat flux, k_(B) is the Stefan-Boltzmannconstant, e_(s) is the surface emissivity, T_(S) is the surfacetemperature, and T_(A) is the ambient temperature. In one embodiment,the surface of the substrate 100 is polished prior to heating. Thisreduces the emissivity and, thus, reduces the energy loss by radiation.

The heat transfer process in a device can be modeled according to theFourier equation $\begin{matrix}{{{C\quad\gamma\frac{\partial T}{\partial t}} + {\nabla{\cdot q_{cond}}}} = Q} & (9)\end{matrix}$where C is the specific heat of the material, y is the mass density, Tis the temperature distribution, q_(cond) is the conductive heat flux,and Q is the heat generation within the material. The solutions to thetemperature distribution and the heat generation can be obtained byapplying initial and boundary conditions. A common initial conditionassumes that the initial temperature distribution is a constant equal tothe ambient temperature. A boundary condition can be obtained using theconservation of energy principle at the surface of the device. Onepossible representation can be written as $\begin{matrix}{{{\lambda\frac{\partial T}{\partial n}} + q_{conv} + q_{{ra}\quad d} + Q_{S}} = 0} & (10)\end{matrix}$where λ is the thermal conductivity of the material, T is thetemperature distribution, n is a direction normal to the surface,q_(conv) is the convective heat flux, q_(rad) is the radiative heatflux, and Q_(S) summarizes any additional surface losses such as thoseincurred if the material is quenched. However, additional boundaryconditions can be found based on geometrical symmetries. Usingcomputerized numerical methods, solutions can be obtained for differentmaterials of varying geometries. In microsystem applications, the modelcan require further refinement to account for phenomena typicallydisregarded on larger scales.

Restating the process, a time-varying current is passed through theinductive coil 170 at an appropriate frequency and power setting, asdescribed above, for a necessary time duration. The magnetic fluxproduced by the coil 170 induces a current in the film 150, resulting inthe generation of heat energy due to resistive and hysteresis losses asdetailed earlier. The process of placing the system 100 near theinductive coil 170 can be repeated to achieve different applicationspecific characteristics. Upon completion of the induction heating, thefilm 150 can be either removed or allowed to remain as part of theoverall structure.

Many devices, such as certain gas-composition sensors, gaspreconcentrators, gas separation columns for chromatography, precisionvoltage or current reference circuitry, thermopneumatic valvestructures, and many others, require heat-generation capability duringnormal operation. The heat generation is often accomplished usingcurrent-generated dissipation by attaching wires to the devices.

FIG. 5 shows an isometric view of a second system 200 including a secondsubstrate 202, a heating element 205, an electronic device 215, and amechanical component 220. The second system 200 is further placed near aset of induction coils 270 positioned in a substrate chamber 274 toachieve a desired magnetic coupling effect represent by 260. Thisarrangement essentially eliminates the need for external leads andfacilitates implementation in locations where physical connections areundesirable or difficult to obtain. The inductive coil 270 near theheating element 205 causes the element 205 to radiate thermal energy.Depending on the application, the element 205 can be located either onthe surface of the device 200 or embedded under one or more layers.

One method for inductive heating a specific region on substrate will nowbe discussed. The acts discussed below can vary for other embodiments.To inductively heat thin films 150 deposited or applied and patterned ona silicon wafer or to locally heat selected regions of an integratedmicrosystem device on a substrate 102, materials are first preferablyprepared. For example, the substrate 102 or the wafer is first preparedwith a thin thermal oxide layer. The thin thermal oxide layer isgenerally used for structural support and substrate anchoring during theprocess. These optional layers of low-pressure chemical vapor deposition(“LPCVD”) oxide have different thickness and are first deposited at apre-selected temperature.

Depending on the design, devices 110 and structures 115, 120 arepositioned on the substrate 102, or inside the substrate 102. Examplesof devices and structures include single- and double-anchored cantileverbeams, folded beams, Guckel rings, vernier strain gauges, sensors, gaspreconcentrators, gas separation columns for chromatography, precisionvoltage or current reference circuitry, thermopneumatic valve structuresand the like. Before the thin film 150 is deposited or applied, thedevices 110 or structures 115, 120 with pre-selected thickness are firstpatterned. Specifically, the devices 110 and structure 115, 120 arepatterned in a caustic or an alkaline solution such as potassiumhydroxide using an oxide hardmask. Alternatively, other device 110 orstructure 115, 120 patterning techniques such as sputter etching,reactive ion etching, ion beam etching, deep reactive ion etching, orother plasma or dry processing techniques may be used. The substrate 102is then covered with a sputter-deposited oxide 124 with anotherpre-selected thickness to prevent them from coming into contact with thethin film 150.

Thereafter, an inductive thin film 150 is chosen. To choose an inductivethin film 150, the permeability and its corresponding Curie temperature,among other things, are considered. For example, cobalt can be used asan inductive thin film 150 because cobalt has a maximum permeability ofapproximately 250 times that of typical microsystem materials and itsCurie temperature indicates efficient heating up to 1115° C., therebyallowing it to provide adequate thermal energy for grain regrowth inpolysilicon. The thin film 150 is typically a ferromagnetic materialthat consists of, but not limited to, one or more of the elements suchas iron, nickel, or cobalt.

In order to prevent the formation of diffused oxides on the substrate102 due to the thin film 150, such as cobalt suicides, a diffusionbarrier 124 (FIG. 2) or a plasma-enhanced chemical vapor deposition(“PECVD”) oxide film is deposited over the entire substrate 102 orwafer. Thereafter, the chosen film 150 with the pre-selected thicknessis evaporated or sputter-deposited onto a side of the substrate 102using techniques such as shadow masking or sputtering with a thin filmdepositor 178 (FIG. 3). In the previous example, a 100 nm-thick cobaltfilm can be evaporated onto one side of the wafer using the shadow masktechnique. Optional positioning of magnets near the substrate 102 toalign the cobalt domains can be performed.

Heat reducers 182 around the targeted devices 110 or structures 115, 120are then optionally and regionally provided to the substrate 102. Heatreducer examples include heat-sink, insulating layer, and thermalbarrier layer. The substrate 102 is covered with the films 150 such thatthe heat generated in the targeted areas remains concentrated due to thethermal conductivity of the substrate 102.

The substrate 102, with the films 150, is then inductively heated in asubstrate chamber 174 (e.g., a vacuum chamber). The substrate chamber174 provides a low pressure ambient with minimal energy loss due toconvection, among other things, thus stabilizing the annealing processand increasing the energy efficiency of the system. In operation, thesubstrate chamber 174 contains a controlled ambient gas. The ambient gascan be inert gas like nitrogen or argon for annealing. Other ambientgases include oxygen, forming gas (a mixture of nitrogen and hydrogen),and ammonia, or others. Depending on the deposition technique chosen,reactive gases can also be used. For example, the class of hydride gasessuch as silane, disilane, dichlorosilane, trichlorosilane, chlorosilane.The substrate chamber 174 can also be a vacuum chamber, such as a NortonNRC-3117 vacuum system, equipped with a mechanical vacuum pump and avacuum diffusion pump. The mechanical pump, such as a Sargeant Welchmodel 1397 pump, is capable of a pressure of approximately 5×10⁻⁴ Torr.The vacuum diffusion pump, such as a Norton type 0162 oil diffusionpump, is capable of an ultimate pressure of approximately 5×10⁻⁸ Torr.The vacuum chamber can include a stainless steel baseplate with aplurality of feedthrough adapters that are configured to introduceelectrical, fluidic, process gas, or mechanical manipulation mechanismsinto the vacuum chamber.

Several coil configurations 170 can be used to apply the inductive heat.Example coil configurations include a solenoidal coil or a spinal coil.The coils 170 receive a varying current that fluxes at a frequency. Anexample frequency is 5 MHz. The solenoidal coil is generally chosen tosubject the substrate 102 to a strong and relatively uniform magneticfield 160 with a dominant component in the z-direction. The substrate102 is then positioned in the coils 170 such that the induced eddycurrents are in the r-φ plane, generally eliminating the dependence ofthe required frequency on the film thickness.

The thermal annealing process is thus conducted at pressures belowapproximately 0.2 Torr, or any pressure that minimizes heat convectionby the gas from the substrate 102. In one embodiment, high purity gas,such as nitrogen or argon, is continuously supplied into the chamber 174using mass flow control, such as a Unit series 1200 mass flowcontroller. The annealing process then begins with controllablyenergizing the coil 170 by passing current from a current-voltage source186 through the coil 170. Depending on the film 150, the size of thefilm 150, the volume of the film 150, the substrate 102, the size of thesubstrate 102, and the volume of the substrate 102 among other thingsselected, the energizing current can be short pulse, medium pulse, longpulse, short continuous or long continuous. The current is then appliedto the coil 170 for a predetermined duration of time. Generally, shortpulses leading to a one-second annealing process is sufficient. However,other durations can also be substituted. The energized coil 170 thenresults in a magnetic flux 160 at the coil 170 thereby inducing acurrent in the thin film 150. The induced currents then generate heatenergy in the substrate 102 due to resistive and hysteresis losses. Theprocess of placing the substrate 102 with the thin film 150 in thevacuum chamber to be near the coil 170 can be repeated to achievedifferent application specific characteristics. Optionally, the thinfilm 150 is then removed from the substrate 102 in an acidic solutionsuch as sulfuric peroxide solution.

In yet another embodiment, a substrate 102, such as p-type siliconwafers, is placed in a furnace with oxygen (O₂) or water vapor (H₂O)ambient that oxidizes the substrate 102. This results in a thin layer ofthermal silicon dioxide (SiO₂) to be grown on the substrate 102.Thereafter, structural anchors are put in place. For example, apatterned layer of thermal oxide that anchors the structures or devicescan be deposited on the substrate 102.

For example, a hexa-methyl-disilane (“HDMS”) or an adhesion primer and alayer of photoresist are spun on the substrate 102. The spinning can beperformed on a spin station such as a Laurell Model WS-400-6NPP/LITEspinning station. In the embodiment, a Shipley S1813 photoresist isused. The substrate 102 is then softbaked at a preset temperature for aspecific amount of time. For example, for a p-type silicon wafer, a ColeParmer Dataplate digital hot plate softbakes the wafer at 90° C. for oneminute. The photoresist is subsequently patterned using a mask such asthe EV620 mask aligner, exposing the photoresist to ultraviolet (“UV”)light. The UV light causes the exposed photoresist long polymer chainsto break down into short chain polymers. Next, the photoresist isdeveloped in a developer solution such as Shipley MF319. The developerdissolves away the broken down photoresist while leaving the unexposedphotoresist intact. The areas where no photoresist is left is thenetched using a buffered acid, such as a 5:1 buffered hydrofluoric(“BHF”) acid.

A device 110 or structure 115 such as a polysilicon is then deposited tothe substrate 102 at a predetermined temperature. For example, 2 micronsof chemical vapor deposition (“CVD”) polysilicon is deposited on thesubstrate 102 using Thermco TMX furnace at 625° C. Thereafter, oxide isagain deposited or sputtered on the structure using a Perkin-Elmer2400-8J RF sputtering system, for example. Similar to the oxidedeposition procedure discussed, both HMDS and photoresist are spun onthe substrate 102. The spun substrate 102 is then softbaked, masked,developed and etched. The polysilicon structure is finally etched in theareas where no oxide is present using another caustic (base) solutionsuch as a 10:1 potassium hydroxide (KOH). Alternatively, other etchingtechniques may be used, such as sputter etching, reactive ion etching,deep reactive ion etching, ion beam etching, plasma etching, and otherplasma or dry processing techniques.

After the device 110 or the structure 115 has been deposited on or inthe substrate 102, a thin film depositor 178 (FIG. 3) deposits a barriermaterial 124, such as an oxide layer, that separates the polysilicondevice 110 or structure 115, 120 from the thin film 150. For example, aPerkin-Elmer 2400-8J RF sputtering system can deposit a 0.25 micronlayer of sputtered SiO₂ on the substrate 102. In some embodiment, theoxide layer is not patterned.

After the separating oxide layer has been deposited, a thin film 150 tobe inductively heated is deposited or applied to the substrate 102. Aferromagnetic material, such as Cobalt, can be used as the thin film.The process of applying the thin film 150 is similar to the depositionof oxide to the substrate 102. Both HMDS and photoresist are first spunon the substrate 102 with the device 110 or structure 115 anchored. Thesubstrate 102 is again softbaked, photoresist masked, and developed.Thereafter, the thin film 150 is applied, deposited or sputtered on thesubstrate 102. Subsequently, masked areas of the thin film 150 above thephotoresist can be lifted using a solvent solution, such as acetone,with a Branson 3210 ultrasonic agitator leaving the thin film 150 on thesubstrate 102 only in the patterned regions.

The substrate 102 with the device 110 and the structure 115, 120 is thenpositioned in a substrate chamber 174, 274 and heated with a Lepel 7.5kW system. The substrate 102 is typically mounted on thermally andelectrically insulating contacts horizontally over the inductive coil170 inside the substrate chamber 174. The mount can be a dielectric,such as fused quartz or silica, configured to incorporate a minimalcontact to the substrate 102. Current is thereafter applied to the coil170, which is positioned near (for example, surrounds or is adjacent to)the thin film. Once the current starts to energize the coil 170, thecoil 170 generates an electromagnetic field having an electromagneticflux as described earlier. The flux induces an eddy current in the thinfilm 150. Due to the resistive nature of the thin film 150, heat isgenerated in the thin film 150. Once the heat reaches a certaintemperature, the substrate 102 is accordingly heated. As a result, thedevice 110 or the structure 115, 120 is thermally annealed or is bondedon or to the substrate 102.

In still another embodiment, the inductive heating of thin films on asubstrate 102 involves the post-CMOS integrated processing of MEMSelements on the same wafer or substrate 102. A high densitymicroelectronic device 105, a high density CMOS process for example, isfirst incorporated into a substrate 102. In some embodiments, thesubstrate 102 is a complete CMOS substrate.

Thereafter, appropriate electrical, mechanical, or optical passivationscan be made to the substrate 102. For example, the substrate 102 can bepassivated by a deposition technique such as sputtering orplasma-enhanced chemical vapor deposition (“PECVD”). Local interconnectfor the post-CMOS device 110, 115, or 120 can be formed on thepassivation layer or layers. For example, a spacer layer of sacrificialmaterial is deposited and patterned for a sacrificial MEMS process. AMEMS structural layer 115, 120 can then be formed on top of the spacer.The spacer layer can be made from a low temperature doped silicondioxide glass like phosphosilicate glass (“PSG”) or borophosphosilicateglass (“BPSG”).

Once the low temperature MEMS structural layer has been deposited ontothe spacer, additional doping of suitable thickness to improve theelectrical performance of the material is required in some embodiments.For example, in a lateral accelerometer structure, the layer thicknessranges from 3 μm to 30 μm. The structural layer is then patterned andetched to form the device geometry appropriate for the design. Thestructural layer 115, 120 can also be passivated for electrical anddopant isolation from any subsequent materials. A suitable electricaland diffusion barrier 124 such as a low temperature silicon nitride willpassivate the structural layer.

An inductive coupling material 150 is thereafter deposited and patternedto form localized regions where the inductive coupling is maximized. Inone embodiment, the thin film 150 thickness ranges from 0.3 μm to 5 μm.In some other embodiments, the thin film 150 is incorporated in thesubstrate 102. Heat reducers 182 are used in areas that are designed toremain cool relative to the MEMS structural material 115, 120.Additionally, the thin film 150 is removed in the areas that aredesigned to remain cool during the annealing process.

The substrate 102, with the devices 110 and the structures 115, 120covered with thin films 150, is subsequently placed in or near aninductive coil 170 within a chamber 174, 274. In one embodiment, thechamber 174, 274 is a vacuum chamber. Once current is applied throughthe coil 170, the coil 170 induces magnetic coupling denoted by 160. Thecoupling further generates an eddy current in the thin film 150, whichin turn generates heat in the substrate 102.

After a localized temperature is reached to affect the change inmaterial properties desired for the MEMS material 115, 120, thesubstrate 102 is removed from the coil 170. In some embodiments, theinductive film 150 is removed from the substrate 102. Material propertychanges induced in the thin film 150 may include an increase inelectrical conductivity due to enhanced diffusion of dopants into thefilm 150 and a reduction in the mechanical stress and stress gradientsin the structural film 115. Furthermore, the passivation between 150 and115 and 120 can also be removed. At this point, the structures 115 and120 can be released using a typical surface micromachining sacrificialetch process, resulting in the completion of the mechanical annealingprocess.

In addition to the above-described method of controllably energizing acoil to generate a magnetic flux that induces current in theferromagnetic thin film to heat it, other ways exist to apply anoscillating magnetic field to the ferromagnetic thin film to inducecurrent therein and heat the ferromagnetic thin film. For example, aresonant cavity can be used, wherein the resonant cavity is a closedrectangular, cylindrical, or spherical metal box that acts as awaveguide and into which an electromagnetic field is introduced in somemanner. A substrate can be positioned in the resonant cavity such thatan oscillating magnetic field is transverse to the surface of thesubstrate. For example, a short probe wire or loop energized from asuitable frequency source and protruding into the cavity can be used tointroduce an electromagnetic field. In one embodiment, a cylindricalcavity having a radius on the order of tens of centimeters can be used,wherein a substrate including a ferromagnetic thin film can be locatedat one end and of the cylindrical cavity and parallel thereto. Amicrowave frequency source or higher wavelength source can be used asthe source of the electromagnetic field and an oscillating magneticfield can be generated that has a desired orientation with respect tothe ferromagnetic thin film such that currents are induced in theferromagnetic thin film and heat is generated.

In another application, the induction heating of a ferromagnetic thinfilm itself is useful to generate the heat required for the magneticannealing of that ferromagnetic thin film. For example, with referenceto FIG. 6, a system 300 is illustrated for performing magnetic annealingof a ferromagnetic thin film 304. In particular, system 300 includes asubstrate chamber 308, a coil 312, a magnetic field source 316, and apower source 320.

As previously described, the substrate chamber 328 can provide a vacuumambient, a low pressure ambient, or can contain a controlled ambientgas. Further, a substrate 324 can be placed in the substrate chamber308. In the illustrated embodiment, the substrate 324 includes aplurality of devices 328 and structures 332 that can include variouslayers that can be fabricated using the techniques previously describedas well as others. In particular, one or more devices 328 or structures332 can include a ferromagnetic thin film 304. Further, one or moredevices or structures can include a first thin film layer 306.

In one embodiment of a method for magnetic annealing, a vacuum ambientis provided in the substrate chamber 328 to reduce the amount ofoxidation of the ferromagnetic thin film 304 during the magneticannealing process.

In order to perform magnetic annealing of the ferromagnetic thin film304, an oscillating magnetic field is applied to the ferromagnetic thinfilm to induce current in the ferromagnetic thin film to heat it.

In one embodiment, a magnetic flux 160 is applied to the ferromagneticthin film 304. For example, the coil 312 is positioned near theferromagnetic thin film 304 and is controllably energized such that amagnetic flux 160 results and the magnetic flux 160 induces a current inthe ferromagnetic thin film 304 thereby heating the ferromagnetic thinfilm 304. As discussed above, coil 312 is selected to have a structure,orientation, and distance from the substrate 324 so as to achieve adesired magnetic coupling effect, represented by 160. For example, coil312 can be energized by passing a time-varying current through the coil312 at an appropriate frequency, power setting, and duration. The coil312 can also be pulsed. The power source 320 provides the electric powerto the coil 312. The magnetic flux 160 produced by the coil 312 inducesa current in the film 304, resulting in the generation of heat energy inthe thin film 304, as described above.

In another embodiment, an oscillating magnetic field is applied to theferromagnetic thin film using a resonant cavity and a microwave orhigher wavelength frequency source.

At the same time as the ferromagnetic thin film 304 is heated, adirectional magnetic field is applied to the ferromagnetic thin film 304by the magnetic field source 316. The strength of the magnetic fieldrequired will depend on the type and thickness of the thin film 304 anda desired magnetic characteristic of the annealed film 304. Typicalvalues for the applied magnetic field strength may be in the range of 60to 1000 Oerstads. The magnetic field source 316 need not be located inthe substrate chamber 308. In one embodiment, the source 316 is apermanent magnet. In another embodiment, the source 316 is anelectromagnet, which can generate a directional magnetic field with anapplied electric DC excitation from the power source 320.

Heating the ferromagnetic thin film 304 at the same time that adirectional magnetic field is applied thus results in a desired magneticcharacteristic of the ferromagnetic thin film 304. For example, magneticdomains in the ferromagnetic thin film are restructured. Specifically,the size of the magnetic domains is increased, resulting in an increasein the magnetic permeability of the ferromagnetic thin film.Simultaneously, a desired orientation of the magnetic dipoles in theferromagnetic thin film 304 is established, with an easy magnetic axisthat is aligned with the direction of the applied directional magneticfield.

Once a desired magnetic characteristic is achieved, the method furtherincludes removing the oscillating magnetic field and allowing theferromagnetic thin film 304 to cool. In one embodiment, this is achievedby ceasing the AC energization of the coil 312.

If a desired easy axis of the ferromagnetic thin film 304 corresponds toa direction that the ferromagnetic thin film 304 is easily heated, itmay be possible for the source 316 to be an electromagnet that includesa coil that can be excited with a time varying power source. In otherwords, a single electromagnet can be excited by a DC source and an ACsource such that the same coil provides the directional magnetic fieldand the oscillating magnetic field and magnetic flux 160.

The enhanced permeability of a ferromagnetic thin film that can resultfrom a magnetic annealing process can make that ferromagnetic thin filmmore efficient in a subsequent induction heating process. For example,magnetic annealing is useful in one method of performing localizedheating of a first thin film on a substrate to increase the efficiencyof a ferromagnetic thin film 304. In particular, a first act of applyingan oscillating magnetic field to the ferromagnetic thin film to induce acurrent therein to heat the ferromagnetic thin film is performed. In oneembodiment, the first act includes controllably energizing a coil 312positioned near the ferromagnetic thin film, wherein the firstenergizing act results in a magnetic flux 160 that induces current inthe ferromagnetic thin film 304 thereby heating the ferromagnetic thinfilm 304.

A directional magnetic field is applied to the ferromagnetic thin film304 at the same time that the ferromagnetic thin film 304 is heated bythe first act of applying an oscillating magnetic field, therebymagnetically annealing the ferromagnetic thin film 304.

A second act of applying an oscillating magnetic field to theferromagnetic thin film to induce a current therein to heat theferromagnetic thin film is performed. In one embodiment, the second actincludes controllably energizing a coil 312 positioned near theferromagnetic thin film, wherein energizing the coil results in amagnetic flux that induces current in the ferromagnetic thin film 304thereby heating the ferromagnetic thin film and the first thin film 306to change a property of the first thin film 306, such as describedabove.

In such a method, the amount of heat produced to magnetically anneal thethin film 304 is less than amount of heat produced by the second act ofapplying an oscillating magnetic field to the ferromagnetic thin film.The first act results in an improved permeability of the thin film 304as compared to the un-annealed ferromagnetic thin film. An increase inpermeability means that subsequently heating the first thin film can beperformed more efficiently and will require less power.

Upon completion of the second act of induction heating, the film 150 canbe either removed or allowed to remain as part of the overall structure.

A magnetic annealing process as described above is advantageous in thatno oven or furnace is required with attendant ramp up and down times andenergy requirements. Further, an oven or furnace does not have theability to selectively heat localized regions on or in the substrate. Amagnetic annealing process using induction heating thus saves time andenergy and provides the ability to selectively heat localized regionsonly.

Various other features and advantages of the invention are set forth inthe following claims.

1. A method of performing magnetic annealing of a ferromagnetic thinfilm applied to a substrate, the method comprising: applying anoscillating magnetic field to the ferromagnetic thin film to induce acurrent in the ferromagnetic thin film thereby heating the ferromagneticthin film; applying a directional magnetic field to the ferromagneticthin film at the same time as the ferromagnetic thin film is heated; andallowing the ferromagnetic thin film to acquire a desired magneticcharacteristic and then removing the oscillating magnetic field andallowing the ferromagnetic thin film to cool.
 2. The method of claim 1,wherein the substrate is positioned in a vacuum chamber during the actsof applying an oscillating magnetic field and applying a directionalmagnetic field.
 3. The method of claim 1, wherein the act of applying anoscillating magnetic field includes controllably energizing a coilpositioned near the ferromagnetic thin film to generate a magnetic fluxthat induces a current in the ferromagnetic thin film.
 4. The method ofclaim 3, wherein energizing a coil comprises pulsing a current throughthe coil.
 5. The method of claim 3, wherein energizing a coil comprisescontrolling the frequency and power of a current provided to the coil.6. The method of claim 3, wherein the acts of energizing a coil andapplying a directional magnetic field are both performed using a singleelectromagnet.
 7. The method of claim 1, wherein the desired magneticcharacteristic is a desired value of permeability.
 8. The method ofclaim 1, wherein the desired magnetic characteristic is a desiredorientation of magnetic dipoles in the ferromagnetic thin film.
 9. Themethod of claim 1, wherein the directional magnetic field is appliedusing one of a permanent magnet and an electromagnet.
 10. The method ofclaim 1, wherein the act of applying an oscillating magnetic fieldincludes using a resonant cavity.
 11. A method of performing magneticannealing of a ferromagnetic thin film, the method comprising: applyinga ferromagnetic thin film to a substrate; energizing a coil with an ACsource at a predetermined frequency, power level and duration togenerate a magnetic flux that induces a current in the ferromagneticthin film thereby heating the ferromagnetic thin film; and applying adirectional magnetic field to the ferromagnetic thin film at the sametime as the ferromagnetic thin film is heated, thereby allowing theferromagnetic thin film to acquire a desired magnetic characteristic.12. The method of claim 11, further comprising positioning the substratein a vacuum chamber prior to the act of energizing a coil.
 13. Themethod of claim 11, wherein the act of applying a directional magneticfield is accomplished by energizing a coil with a DC source.
 14. Themethod of claim 13, wherein the same coil is used for both acts ofenergizing a coil.
 15. The method of claim 11, wherein the magneticfield is applied using one of a permanent magnet and an electromagnet.16. A method of performing localized heating of a first thin film on asubstrate, the method comprising: applying a ferromagnetic thin film tothe substrate; performing a first act of applying an oscillatingmagnetic field to the ferromagnetic thin film, the first act inducingcurrent in the ferromagnetic thin film thereby heating the ferromagneticthin film; applying a directional magnetic field to the ferromagneticthin film at the same time that the ferromagnetic thin film is heated bythe first act of applying an oscillating magnetic field, therebymagnetically annealing the ferromagnetic thin film; and performing asecond act of applying an oscillating magnetic field to theferromagnetic thin film, the second energizing act inducing current inthe ferromagnetic thin film thereby heating the ferromagnetic thin filmand the first thin film to change a property of the first thin film. 17.The method of claim 16, further comprising positioning the substrate ina vacuum chamber prior to the first act of applying an oscillatingmagnetic field.
 18. The method of claim 16, wherein the ferromagneticthin film is patterned to selectively control which areas on thesubstrate are heated.
 19. The method of claim 16, wherein the first thinfilm does not combine with the ferromagnetic thin film.
 20. The methodof claim 16, further comprising removing the ferromagnetic thin filmafter changing a property of the first thin film.
 21. The method ofclaim 16, further comprising applying a diffusion barrier between thefirst thin film and the thin ferromagnetic film.
 22. The method of claim16, wherein at least one of the first and second acts of applying anoscillating magnetic field to the ferromagnetic thin film includescontrollably energizing a coil positioned near the ferromagnetic thinfilm to generate a magnetic flux that induces a current in theferromagnetic thin film to heat the ferromagnetic thin film.
 23. Themethod of claim 22, wherein controllably energizing a coil comprisescontrolling the duration and frequency of a current provided to thecoil.
 24. The method of claim 16, wherein the act of applying anoscillating magnetic field includes using a resonant cavity.